drm/atomic-helper: Add atomic_mode_set helper callback

Some encoders need more information from crtc and connector state or
connector display info than just the mode during mode setting. This
patch adds an atomic encoder mode setting variant that passes the crtc
state (which contains the modes) and the connector state.

atomic_enable/disable variants that additionally pass crtc and connector
state don't seem to be necessary for any current driver. mode_fixup
already has an atomic equivalent in atomic_check.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>